1. Field of the Invention
The present invention relates to an organic electroluminescence type display apparatus and a method of manufacturing the same. In particular, the present invention relates to an organic electroluminescence type display apparatus in which an organic electroluminescence (EL) device (hereinafter, referred to as “organic EL device”) is formed as an electro-optic device on an active matrix type substrate having a thin film transistor (TFT) formed thereon, and to a method of manufacturing the same.
2. Description of Related Art
In recent years, an organic electroluminescence type display apparatus in which a light emitter such as an organic EL device is used as an electro-optic device has been commonly used as one of display panels. An organic EL device has a basic structure in which an electroluminescence layer including an organic EL layer is interposed between an anode electrode (also referred to as “anode”) and a cathode electrode (also referred to as “cathode”). When a voltage is applied between the anode and the cathode, holes are injected from the anode side and electrons are injected from the cathode side, with the result that the organic EL layer emits light (see, for example, Japanese Patent No. 2597377 (Steven et al.)).
The organic electroluminescence type display apparatus which is a display apparatus incorporating such an organic EL device includes a TFT active matrix substrate on which a thin film transistor (hereinafter, referred to as “TFT”) serving as a switching element is disposed. The organic EL device in which the anode, the electroluminescence layer, and the cathode are stacked on each other in this order is formed in each pixel of a display panel area on the active matrix substrate.
Examples of the conventionally commonly used organic electroluminescence type display apparatus include a so-called bottom emission type. In the organic electroluminescence type display apparatus of the bottom emission type, a TFT and an organic EL device are formed on a transparent insulating substrate such as a glass substrate. To emit light, which is generated from the organic EL layer of the organic EL device, to the back side of the transparent insulating substrate on which the TFT is not formed, the anode is made of a conductive material having a light-transmitting property, such as indium oxide In2O3+tin oxide SnO2 (hereinafter, referred to as “ITO”).
In order to increase the efficiency of hole injection into the organic EL layer, it is regarded as preferable that the anode be made of a conductive material having a high work function value. For example, Steven et Al. discloses that the preferable work function value for the anode is equal to or larger than 4.0 eV. ITO is preferably used as the material of the anode, because ITO has a work function value of about 4.7 eV.
In the bottom emission type, however, light is not allowed to be transmitted through a region in which a TFT pattern and a wiring pattern, which are formed on the substrate, a pattern of a signal driving circuit, and the like are formed. This causes a problem of a reduction in effective light-emitting area. To solve the problem, a structure called a top emission type capable of securing a wide light-emitting area has been developed.
In the organic electroluminescence type display apparatus of the top emission type, the anode is made of a metal material having light reflectivity. Accordingly, when the light generated in the organic EL layer is allowed to be transmitted through the cathode and is emitted to the upper portion of the substrate, the reflected light that is reflected on the anode made of the metal material can be emitted to the upper portion of the substrate at the same time. Thus, a bright display image can be obtained.
In order to obtain a high luminous efficiency and a bright image in the organic electroluminescence type display apparatus of the top emission type, the anode needs to have a high work function value and a high light reflectance. In view of patterning workability, Cr (about 4.5 eV) and Mo (about 4.6 eV), for example, can be selected as the metal material having a high work function value. However, those metal films have a problem in that the light reflectance is low and the loss of the reflected light is large when the films are applied to the anode. According to the measurements made by the inventors of the present invention, the light reflectance at a wavelength of 550 nm is 67% for Cr and 60% for Mo. Moreover, those materials have a high specific resistance value of 10 μΩ·cm or larger and a low hole injection efficiency, which causes a problem of a reduction in luminous efficiency.
On the other hand, as examples of the metal material having a high light reflectance of 90% or higher and a low specific resistance value of 10 μΩ·cm or smaller, Al, Ag, and an alloy of those metals can be selected. However, those metal films have a work function value smaller than the preferable work function value, that is, 4.0 eV, which causes a problem in that it is difficult to increase the luminous efficiency of the organic EL device.
To solve the above-mentioned problem, there is disclosed an anode which has at least a two-layer structure, in which a conductive material having a high work function is stacked on Ag or Al having a high light reflectance and a low specific resistance value, or an alloy thereof, and which has a structure satisfying both the high reflectance and the high hole injection efficiency (see, for example, Japanese Unexamined Patent Application Publication Nos. 2001-291595, 2003-77681, 2003-288993, and 2004-31324). Examples of the conductive material having a high work function include a metal oxide thin film. Many of them have a light-transmitting property and are advantageous in that the light reflectance of the metal of the lower layer does not greatly deteriorate.
When an Ag film or an Ag alloy film is used as the material having a high reflectance, however, those materials are chemically extremely active, and the reflectance greatly deteriorates with age due to the surface oxidation in the atmosphere. For this reason, it is difficult to apply those films to the real process.
Further, when the transparent conductive oxide film is formed as an upper layer on a metal film or an alloy film constituting a light reflective film as a lower layer, an oxide reaction layer including metal atoms of the lower metal film and oxygen contained in the upper layer is newly formed on an interface between the upper layer and the lower layer. The oxide reaction film is formed of an electrically insulating material, which causes a well-known problem that the resistance value of the device is abnormally increased as disclosed in Japanese Unexamined Patent Application Publication Nos. 04-253342, 06-196736, and 2000-77666.
FIG. 6 shows results of an experiment conducted by the inventors of the present invention on the dependency of the material of the metal film on the contact resistance between the metal film and the light-transmitting conductive oxide film. As is apparent from the graph shown in FIG. 6, the electrical contact resistance value on the interface in the case where the Al film and the Al alloy film, which is a light-transmitting conductive oxide film, according to a related art example are stacked on each other is extremely high, that is, the electrical contact resistance value of the related art example is 107 times as high as that of the case where Cr and the ITO film are stacked on each other according to Comparative Example 1, assuming that the value of the case where Cr and the ITO film are stacked on each other is 1.
Also in the case where the anode has a structure in which the ITO film is merely stacked on the Al alloy film, which is a lower layer, the oxide reaction layer, which is formed of an electrical insulating material, is formed on the interface. As a result, there is a problem in that the efficiency of hole injection into the organic EL layer remarkably deteriorates. In the case of the Al film, there is another problem in that surface irregularities such as hillock are more likely to occur, and a short-mode failure with the opposing cathodes sandwiching the organic EL layer and defective display called a dark spot are more likely to occur. The above-mentioned technologies fail to disclose any countermeasures against such problems. Therefore, it is substantially impossible to apply those technologies to the real device.
As a method for solving those problems, Japanese Unexamined Patent Application Publication No. 2004-214606 (Gotoh et al.) and Japanese Unexamined Patent Application Publication No. 2004-363556 (Ikeda et Al.), for example, disclose a method of reducing the electrical contact resistance with the transparent conductive film by adding Fe, Co, Ni, or the like to Al.
According to the methods disclosed in Gotoh et al. and Ikeda et al., for example, in an AlNi alloy in which Ni is added to Al, the formation of an oxide of Al on the interface of Al is prevented to obtain excellent contact resistance with the transparent conductive film. However, as a result of the experiment, the inventors of the present invention have found that there arises a problem that the transparent conductive film, for example, indium oxide of ITO is reduced and a part of the metal In separates out and becomes a dark spot, which results in defective display.
The present invention has been made in view of the above-mentioned circumstances, and therefore, an object of the present invention is to provide an organic electroluminescence type display apparatus capable of enhancing a luminous efficiency and obtaining a bright display image, and a method of manufacturing the same.